1. Field of the Invention
The field of invention relates to zeolites. More specifically, the field relates to partially collapsed zeolites for the purification of hydrocarbon based gaseous fractions such as natural gas.
2. Description of the Related Art
Almost one quarter of the total worldwide production of energy is met through natural gas production. The regulations for the transportation of natural gas that occurs mainly through pipelines vary by country. In many countries and jurisdictions, there are specific restrictions to the amounts of inert chemical species such as nitrogen (N2) and carbon dioxide (CO2) that may be transported. Nitrogen is typically found in wellhead gas in a concentration range of about 0.5 to 5 mole percent and may approach concentrations of up to 30 mole percent. Sub-quality natural gas is a composition that exceeds pipeline specifications for contaminants such as CO2, hydrogen sulfide (H2S) and nitrogen. For instance, sub-quality natural gas often has a nitrogen concentration exceeding 4 mole percent and a CO2 concentration in a range of about 0.2 mole percent to about 1 mole percent with respect to the wellhead gas. Both nitrogen and CO2 have no heating value and therefore reduce the thermal quality of the wellhead gas. In addition, CO2 is an “acid gas” that, in the presence of water, forms carbonic acid. The resulting acid reacts rapidly with carbon steel and other metals susceptible to acidification and produces corrosion, a common problem in areas along a pipeline where pools of aqueous liquids may form.
CO2 is normally removed during natural gas refinement and processing by the process of amine scrubbing using gas-liquid contactors operating at a temperature range of from about 323 K to about 333 K. The resulting (saturated) alkanolamine is regenerated in a temperature range of from about 383 K to about 403 K and releases the purified carbon dioxide. This energy intensive process typically involves the handling of a corrosive and toxic solvent. In addition, the removal of nitrogen from methane, the primary component in natural gas is very difficult. The only commercial process commonly used for separating nitrogen from methane is cryogenic distillation, where a turboexpander reduces the temperature of the gas to about 220 K. The nitrogen-poor product stream must be recompressed to transport it through pipelines effectively. Both turboexpansion and recompression are energy-intensive and therefore increase the costs associated with natural gas processing.
Adsorption processes using zeolites are capable of performing certain CH4—CO2 and CH4—N2 separations. For instance, Molecular Gate® (Engelhard Corp.; Iselin, N.J.) uses titanosilicate-based zeolites (ETS and CTS configurations) doped with transition metals that allow for the micropores of the zeolite to be adjusted based upon activation temperature. Other adsorbents include carbon based molecular sieves for CH4—N2 separations. A pressure swing adsorption (PSA) system using metal-exchanged clinoptilolites, a natural zeolite largely comprised of silica and alumina, has also shown some promise for CH4—N2 separation. In addition, CMS 3A (carbon molecular Sieve 3A) has been evaluated for performing CH4—CO2 separation.
As a selective adsorbent of N2 and CO2, zeolite-based materials are attractive candidates. Zeolite 13X, which is an aluminosilicate zeolite, has been shown to reduce carbon dioxide levels in flue gases at low temperatures. Zeolites are thermochemically stable, available in the market and their surfaces can be controlled through post-modifications such as ion-exchange. Zeolites have well-defined microporous structures with mean diameters in a range of from about 0.3 nanometers (nm) to about 1.5 nm, allowing a zeolite material to advantageously provide a molecular sieve type effect for separating certain unwanted constituents found in natural gas.
Despite the advantages of zeolites, the separation of N2 and CO2 from CH4 remains challenging. For instance, the extremely small difference between the kinetic diameters of the compounds (CO2: 0.33 nm; N2: 0.36 nm; CH4: 0.38 nm) requires precision in forming zeolite apertures. It should be noted that the pore diameter of zeolites and similar materials is difficult to control in the ultra-small pore range (e.g. materials with mean diameters less than 0.38 nm). The attraction of a titanosilicate-type ETS-4 zeolite for small molecular separations is attributable to its pore size tuning. However, two significant problems are associated with the broad use of titanosilicate materials: 1) they have lower thermal stability, so it is more difficult to use them in processes that apply thermal cycling to promote adsorption aid desorption; and 2) these materials can be costly and not readily available. In this regard, aluminosilicate-based zeolites are advantageously more commercially available and less expensive than titanosilicate-based zeolites.